Guide
Cooling Tower Scaling: Causes, Prevention & Treatment
Scale in cooling towers reduces heat transfer, blocks fill, and increases energy costs. Learn about the types of scale, how to prevent it, and how to clean it when it forms.
8 min readUpdated July 2026
Types of Scale
| Scale Type | Chemical Formula | Cause |
|---|---|---|
| Calcium carbonate | CaCO₃ | Most common. Forms when calcium and alkalinity concentrate beyond solubility. |
| Calcium sulphate | CaSO₄ | Forms at very high cycles of concentration. Harder to remove than carbonate. |
| Silica | SiO₂ | Forms when silica exceeds 150 ppm. Extremely hard, almost impossible to chemically remove. |
| Iron oxide | Fe₂O₃ | From corrosion products or iron in the make-up water. Brownish-red deposits. |
| Magnesium | Mg(OH)₂ | Forms at high pH. Less common than calcium scale. |
Why Scaling Occurs
As water evaporates in the cooling tower, the dissolved minerals concentrate. When the concentration exceeds the solubility limit (which depends on temperature, pH, and the specific mineral), the mineral precipitates as solid scale on surfaces.
The solubility of calcium carbonate decreases as temperature increases — so scale forms preferentially on hot surfaces (like heat exchanger tubes) and in the tower fill where the water is warmest.
- High cycles of concentration — too many cycles concentrate the minerals beyond solubility
- High pH — alkaline water promotes calcium carbonate precipitation (above pH 8.5)
- High temperature — calcium carbonate is less soluble at higher temperatures
- Inadequate blowdown — insufficient blowdown allows concentration to exceed the limit
- Hard make-up water — high calcium/magnesium in the source water
Consequences
- Reduced heat transfer — scale is an insulator. 1mm of calcium carbonate reduces heat transfer by approximately 10%.
- Blocked fill — scale blocks the air passages in the fill, reducing airflow and cooling capacity.
- Increased energy consumption — a scaled tower requires more fan and pump energy to achieve the same cooling.
- Reduced water flow — scale in the distribution nozzles blocks water flow, creating dry spots in the fill.
- Microbiological growth — scale provides a rough surface for biofilm attachment, increasing Legionella risk.
- Reduced cycles — scaling forces lower cycles of concentration, increasing water and chemical consumption.
Prevention
- Control cycles of concentration — keep within the limit for your make-up water hardness. Use a conductivity controller.
- Dose scale inhibitors — phosphonates and polymers keep calcium in solution by interfering with crystal formation.
- Control pH — acid dosing (sulphuric acid) lowers pH and keeps calcium carbonate in solution. Requires careful control.
- Side-stream filtration — removes suspended particles that act as nucleation sites for scale crystals.
- Soft make-up water — if the source water is very hard, consider softening the make-up water.
- Monitor the Langelier Saturation Index (LSI) — a calculation that predicts whether the water will scale or corrode. Target LSI between -0.5 and +0.5.
Cleaning Scaled Towers
- Chemical descaling: Circulate a mild acid solution (diluted hydrochloric or sulphamic acid) through the tower. The acid dissolves calcium carbonate scale. Requires corrosion inhibitor to protect metal components. Follow with neutralising rinse.
- Physical cleaning: For severe scaling, remove the fill media and clean it with high-pressure water or replace it. Scale on the basin can be scraped or brushed.
- Replace the fill: If the fill is heavily scaled and cleaning is not effective, replace it. Scaled fill is structurally weakened and may collapse.